Vaccinated Too Early

Hi there!
So I have a fun story- I turned 18 recently so I'm fostering three kittens and a mama cat before I go off to college. They were assumed to be about one week when I took them. They told me to come back August 19-21 for vaccinations, but my mother INSISTED it was this week. So of course, I trusted her and took them about an hour ago. The people at the shelter didn't even check to see if it was the correct dates. The vet was hesitant about their age so she decided to only give them a half dose. After administering the vaccine they realized that they were, in fact, a few days/almost a week too young. She told me to keep a close eye on them IN CASE something goes wrong, but it's unlikely that something will considering it was only a half dose and they were just barely too young.
I think I'm definitely freaking myself out over this. She said they'll most likely be fine, but I'm terrified. What should I be looking out for? The lady said swelling at the site of the shot and possibly lethargy. At this point I'm not sure the difference between normal kitten sleeping and lethargy. Help me calm down about this please. Is there anything I could do to prevent something from happening?

Thanks for fostering the kittens! You’re making a big difference for them!

Regarding the vaccinations, if the vet said that she’s not concerned, then you shouldn’t be either. I believe that allergic reactions to vaccines (trouble breathing, fainting, etc.,) would be more immediate after the vaccine, so I think you should be in the clear there. But definitely keep monitoring them. This article from the Cornell vet school explains some more signs to look out for: Feline Vaccines: Benefits and Risks

And, yes, kittens do sleep a lot, so I understand it can be tricky to tell when one is lethargic. If they’re sleeping, but still alert, then I think they’re ok. But if they are asleep and don’t respond to being petted, or picked up, then I would call the vet.

First, your vet is excellent for giving them a half dose based on their size and estimated age.
For now, don't work yourself up. Just keep an eye on them. Sometimes grown animal act a little lethargic after getting their shots, but with their age I woudn't hesitate to call the vet if they act off and you're worried.
Do you remember what vaccination they got, was it a single or a combo?
I'm sure they'll be fine, but you're right to keep watch on them.

Sometimes kittens feel a bit puny for a couple of days after vaccines and can eat less or vomit or have diarrhea. Unless symptoms are severe they usually get through it fine!

I wouldn’t worry too much about a vaccine reaction, other than a little sleepiness. If they feel hot, you need to take them back.

Also, next time you take them to the vet, ask whether they were too young for the vaccine to produce the intended immune reaction. They may need to be re-vaccinated. The reason is that the immune system has to be mature enough to produce long-term protective antibodies in response to the vaccine. That’s why vaccines are not often given to newborns.